Primary batteries, such as an alkali dry battery and a nickel dry battery, and secondary batteries, such as a lithium ion secondary battery, are used in various electronic apparatuses. In particular, the lithium ion secondary battery (hereinafter simply referred to as “lithium ion battery”) is used as a power source for a portable information terminal apparatus such as a cellular phone. The lithium ion battery is light in weight and can be used for a long period because large energy can be stored therein. Further, the lithium ion battery has high energy density, has excellent self-discharge properties (energy retention properties), and substantially has no problem of inter-electrode short-circuit due to deposition of dendrite substances. Therefore, the lithium ion battery is used as a power supply not only for the terminal apparatus but also for a hybrid automobile and the like. It is also expected that the lithium ion battery will be mounted on an electric automobile now under development.
In general, various batteries including the lithium ion battery has high sealing properties. In some cases, battery internal pressure abnormally rises because of overcharge, charge due to reverse mounting, or the like. As precautions for such a case, to prevent a burst of the batteries, safety valves for discharging gas as a cause of an internal pressure rise to the outside of the batteries are usually provided in the batteries.
However, when the safety valves do not normally operate, a burst of the batteries is likely to be caused. In particular, for example, a burst of the lithium ion battery expected to be mounted on an automobile could be a cause of fire.